KIEV, Ukraine – Ukrainians braced for invasion and pleaded for help from the United States after their political leaders said Thursday that nearly 100,000 Russian troops were lined up along the eastern Ukrainian border.

"I think an invasion is possible," said Kateryna Zawada, 24, who lives in Donetsk, a city close to the border. "But if it happens, I won't leave Donetsk. Among my friends, many are saying they are ready to fight, if needed."

A week after Russia officially annexed Crimea from Ukraine, following what the West called an illegal and illegitimate referendum, the United States called on Russia to de-escalate aggression toward Ukraine as more troops moved toward the border.

Since pro-Russian forces moved into Crimea after the ouster of Ukrainian President Viktor Yanukovych at the end of February, tensions have been rising in the east of the country bordering Russia, along with fears that President Vladimir Putin will invade.

Russia's defense minister, Sergey Shoigu, has said the troops are at the border for exercise purposes and did not intend to cross. U.S. Department of Defense spokesman John Kirby said Thursday that regardless of their intent, the presence of Russian soldiers at the border did "nothing to de-escalate tension in Ukraine."

The head of Ukraine's Council for National Defense and Security, Andriy Parubiy, said the troops on the border have been in a state of readiness for several weeks.

"We understand that any night, any hour the invasion of Ukraine's mainland can begin," he said. "We're preparing for it."

"There is an increasing likelihood that they (Russia) will actually intervene in Ukraine," said William Pomeranz, deputy director of the Kennan Institute for Advanced Russian Studies at the Woodrow Wilson Center in Washington.

"They want to intimidate Ukraine and have influence over how it organizes its politics going forward and the drafting of the Ukrainian constitution," he said.

Yanukovych, who fled to Russia last month after a warrant was issued for his arrest in the shooting deaths of more than 80 protesters, called for people in eastern Ukraine to hold a referendum on whether to secede and join Russia.

Russia annexed Crimea shortly after pro-Moscow politicians there called for a referendum to secede.

Yanukovych urged Ukrainians to demand a "referendum that would determine the status of each region in Ukraine" in a statement carried by the ITAR-Tass news agency. Yanukovych didn't specify when or how the vote should be held.

The remarks prompted Ukrainian prosecutors to open an investigation against Yanukovych on charges of calling for overthrowing the country's constitutional order. Yanukovych's political rival, former prime minister Yulia Tymoshenko, accused Yanukovych of being "a tool aimed at destroying the independence of Ukraine."

Tymoshenko is running in Ukraine's next presidential election, scheduled for May 25.

Rep. Buck McKeon, R-Calif., chairman of the House Armed Services Committee, said Russia's war on Ukraine "has already started."

"It is time to stop speculating about possibility and start dealing with reality," McKeon said. "Continued inaction by the president in the face of Mr. Putin's invasion will make further Russian aggression more — not less — likely. Any show of resolve from the White House will have my full support."

President Obama has repeatedly warned that there will be "costs" to Russia if it invades, but he has said he intends no military response. NATO, the U.S.-European defense alliance, has not been requested to prepare any military action, and U.S. Defense Secretary Chuck Hagel said Russian Defense Minister Sergei Shoygu assured him "they had no intention of crossing the border into Ukraine."

Ukraine has sought small arms and ammunition, but that has been refused. The Pentagon is sending food.

"The rations, the Meals Ready to Eat, they are on the way," Rear Adm. John Kirby, a Pentagon spokesman, said. "We expect them to arrive in Ukraine probably by the weekend is the best estimate."

Divisions exist between Ukraine's Russian-speaking eastern regions, where many favor close ties with Moscow, and the Ukrainian-speaking west, where most want to integrate into Europe. Many eastern Ukrainian cities have people on both sides.

Russia has issued statements calling for the federalizing of Ukraine and giving its regions more autonomy. Ukraine's government says provinces have no right to secede, according to the country's constitution, and that included Crimea.

The European Union and the United States have imposed travel bans and financial sanctions on Russian and Crimean officials involved in the annexing of the peninsula, but Pomeranz said he did not know what more they could do, outside of significantly increasing sanctions if Russia were to invade.

"The West's options are very limited because it does not appear that we have an immediate military option in eastern Ukraine, and I think the only way the West would be able to respond would be via sanctions and economic pressure," he said. "I think it is highly unlikely at this stage that NATO would engage directly in what is going on in Ukraine. It would immediately escalate the crisis to a major war between Russia and NATO, which I don't think western Europe is looking for."

Some Ukrainians, such as Zawada, said they were ready for a potential invasion. Others weren't willing to fight troops coming in. Anzhela Pisanaya, 44, said she intended to leave her home in the city of Luhansk in the far east of Ukraine.

An ethnic Russian, Pisanaya has supported EuroMaidan, the name given to Ukrainians who protested Yanukovych and oppose Russian influence.

"There is no way that we will be living here under occupation," she said. She put her house on sale and planned to move to Kiev or another region farther within Ukraine where the Russians would not go.

"All we can think about now is how to leave as fast as possible," she said.